1. Field of the Invention
This invention relates, in general, to printing and more specifically to controlling the heat dissipation of electrical components used in printing apparatus.
2. Description of the Prior Art
Dot-matrix printers are available in several different types, including wire or pin impact printers, thermal printers, ink jet printers, and printers which use special laser diode arrays to write images onto a photosensitive member. Although the basic form of these printers is different, each can use a printhead which has more than one dot producing or defining device in the printhead which is electrically controlled or activated. For most printing applications, less than the full number of devices will be activated at every possible position, or option, during the printing operation. However, this cannot usually be guaranteed and some means must exist which protects the electrical components supplying and controlling the printhead from excessive heat dissipation. For economy reasons, most printing apparatus is designed to have electrical components capable of withstanding the head dissipation required in normal printing applications. When the requirements become more demanding, the printers usually have some means for compensating for the extra heat which would be generated by these stringent requirements. This allows the heat dissipating components to be safely sized at the more normal usage requirements.
One method of controlling the heat dissipation is to slow the effective head speed by stopping the printhead momentarily at the end of a line, printing the required dots with more than one pass of the printhead or changing the velocity of the head as it moves across the imaging or printing media. A dot counting technique is frequently used to predict, or calculate in real time, the number of dots to be printed during a predetermined time interval or during a predetermined length of line scan of the printhead. If the calculated or measured values exceed a limit value, the printhead speed is effectively slowed. While this type of head dissipation control is effective in some applications, conventional dot counting does not provide for the most accurate control of excessive heat buildup in critical components of the system. This is so because conventional counting techniques inherently produce an average value for the electrical current which is used to produce the heat. In actual operation, the waveform of the current or power supplying the printhead is irregular and the more exact predictor of heat producing power would be a method which considers the RMS value of the current being monitored. U.S. Pat. No. 4,653,940, issued on Mar. 31, 1987, teaches a dot counting system for use with a dot-matrix printer. In this patent, a technique is used to prevent an abnormal rise in temperature of the printhead when dots are formed continuously with a relatively high density. A fixed interval of time is determined wherein a known heat dissipation rate is assumed. The rate is equated to dot counts and a running tabulation of counts is processed which subtracts a fixed number of counts from the running total during every passage of the fixed time interval. The system purportedly compensates for a known rate of heat loss from the apparatus during the counting sequence, although the calculations are based upon average power levels rather than more accurate power levels determined from RMS currents.
Therefore, it is desirable, and an object of this invention, to provide an efficient power controlling system for dot-matrix printheads which uses RMS current quantities and calculations for determining the need to slow the printhead speed. It is also desirable, and another object of this invention, to provide a system wherein current fluctuations in the supplied power, which are not seen by or influence the heat buildup in the protected components, are not unnecessarily used to influence the calculations made by the controlling method.